1. Field of the Invention
The present invention relates to an electrophotographic printer and an electrophotographic printing method, and more particularly, to an electrophotographic printer, provided with an intermediate transfer drum, and an electrophotographic printing method.
2. Description of the Related Art
Electrophotographic apparatuses, e.g., electrophotographic printers, are designed so that electrostatic latent images formed on a photosensitive medium are developed by means of toners, and the resulting toner images are heated under pressure and fixed on a recording medium, such as paper, by using a heating roll or other transfer means.
Some of these electrophotographic apparatuses are provided with intermediate transfer means, such as a belt or drum, which can transfer the toner images to various recording media, including paper, plastic films, metal sheets, etc.
The conventional electrophotographic apparatuses furnished with the intermediate transfer means include, for example, a transfer-fixing device described in Unexamined Japanese Patent Publication (KOKAI) No. 50-23234 (U.S. Pat. No. 3,955,530) and an electrophotographic toner transfer and fusing apparatus described in Examined Japanese Patent Publication (KOKOKU) No. 57-20632 (U.S. Pat. No. 3,893,761). The former is a wet type which uses liquid toners, while the latter is a dry type which uses dry toners. In the latter type, moreover, a belt is used as the intermediate transfer means. An electrophotographic printing machine described in Unexamined Japanese Patent Publication (KOKAI) No. 63-34573 (U.S. Pat. No. 4,708,460) is known as an example of the wet type which uses a belt as the intermediate transfer means.
The wet electrophotographic apparatuses of the liquid-toner type, which can use minute toner particles of submicron order, have an advantage over the dry type in being able to produce sharper images of higher resolution.
In making a color print by using one such electrophotographic apparatus, an electrostatic latent image corresponding to one color, e.g., magenta, is formed on the photosensitive medium, and is developed by means of a magenta liquid toner. Then, the resulting toner image is transferred to the intermediate transfer means, e.g., an intermediate transfer drum, which is pressed against the photosensitive medium. Thereafter, cyan and yellow toner images are successively transferred, in layers, to the intermediate transfer drum in like manner. Subsequently, transfer-fixing means, such as a heating roll, is pressed against the intermediate transfer drum, whereby the toner image on the transfer drum is transferred to and fixed on paper or some other recording medium, thus forming the color print.
Thus, according to this conventional electrophotographic printer or electrophotographic printing method, the color print is produced by transferring to and fixing the toner image on the recording medium by means of the intermediate transfer drum. To shorten the time for the production of the color print, in this case, the transfer of a toner image for the last color to the intermediate transfer drum is preferably effected by concurrent processing such that the heating roll is pressed against the transfer drum to fix the toner image on the recording medium while the toner image for the last color is being transferred.
When the heating roll is pressed against the intermediate transfer drum, the transfer drum is subjected to a stress such that the force of pressure between the photosensitive medium and the transfer drum is disturbed, depending on the direction in which the heating roll is pressed. Thus, the force of pressure between the intermediate transfer drum and the photosensitive medium varies, so that the toner images transferred to the intermediate transfer drum are battered and become unclear.
According to the electrophotographic printer or electrophotographic printing method of the wet-developing type, liquid toners of different colors are mixed and soiled by one another, so that the resulting color print is liable to be lowered in quality, e.g., in clearness. If the soiling between the liquid toners is prevented by modifying the construction, the electrophotographic printer will inevitably be complicated in structure and large-sized, thus failing to comply with users' request.